1. Field of the Invention
The present invention generally relates to the field of assessing hearing capacity in human beings. The invention, more particularly, relates to systems and methods for determining the hearing ability of an individual by providing an acoustic stimulus in combination with electrophysiological threshold measurement techniques. The invention, still more specifically, relates to systems and methods for determining the hearing ability of an individual through the application of an acoustic stimulus provided by a hearing aid.
Newborn hearing screening programs typically recommend that newborn infants who have failed an initial hearing screening test be sent for a complete hearing assessment prior to their 3-month anniversary. However, since children are unable to produce appropriate motor responses, which are typically required for the behavioral hearing assessment, an objective estimation of hearing is imperative. This objective estimation can be provided by electrophysiological means and has shown excellent correspondence with behavioral thresholds collected later in life. One such electrophysiological test is termed the Auditory Brainstem Response (ABR).
The ABR primarily consists of five waves, labeled I to V, which represent the evoked synchronous activity generated by the auditory nerve and the brainstem. The ABR is evoked by short-duration auditory stimuli with short rise times such as auditory clicks or tone bursts. In order to obtain a waveform that can be interpreted, the evoked response is generally recorded for 15 to 25 ms following onset of the auditory stimulus, averaged over thousands of trials and filtered to eliminate unwanted neuro-muscular or environmental electrical activity. The ABR is recorded from electrodes, which are strategically placed on or in the vicinity of the forehead and mastoids, creating differentials in potential (i.e. dipoles). The evaluation of latency, amplitude and morphology of the ABR wave V will assist the audiologist in determining hearing ability or threshold of the individual. The ABR threshold is associated with the lowest level of auditory stimulation capable of evoking a visible or detectable wave V.
Another frequently used electrophysiological test is the Auditory Steady-State Response (ASSR). The ASSR relies on modulating a pure-tone, either in frequency or amplitude, and presenting this modulated tone to the ear of the patient at various intensities. The ASSR consists of a statistical averaging which determines whether the tone modulation is also represented in the continuous encephalographic activity. Unlike the ABR, the ASSR does not require a time-locked averaging to the onset of the acoustic stimulus, as the modulation and hence the occurrence of the modulation is steady-state. The statistical presence of the modulation at various levels will assist the audiologist in determining hearing ability or threshold of the individual. The ASSR threshold is associated with the lowest level of auditory stimulation capable of evoking its attributed modulation within the measured neural activity.
2. Prior Art
EP-B1-1089659 shows a method for determining an auditory brainstem response (ABR) to an acoustic stimulus in a human test subject.
EP-B1-1133898 teaches a hearing aid for in-situ fitting, where an audiogram is measured with the hearing aid placed in the ear and acting as an audio signal source.
In-situ audiometry has been documented to reduce variability otherwise associated in using one transducer to couple audiometric equipment to the child's outer ear during the evaluation phase and another transducer used to couple the hearing aid to the child's outer ear during the amplification phase. In so doing the coupling and/or tubing and/or venting properties of each transducer are difficult to estimate during the calculation of output characteristics for the child's hearing aid in relation to the measured hearing loss. There is therefore a significant advantage of using the same coupling system and identical transducer both for the evaluation of hearing loss and the subsequent calculation of output characteristics for the hearing aid, such as prescribed by an in-situ technique.
At the present time, there are several ABR and ASSR equipments on the market. These equipments both deliver the acoustic signals to the individual's ears using headphone or insert earphone transducers and collect and manipulate the electrical activity generated by the auditory nerve and brainstem.
While these tools are useful in determining the unaided thresholds of children and adults, they are somewhat limited in their applicability towards common intervention or treatment practices in two important ways: a limitation to determine an aided threshold and a limitation to consider the disparity between the influence of the transducers used in the evaluation and treatment phases of an intervention with a hearing-impaired individual.
Aided thresholds reveal important information to the audiologist as they reveal the softest sound that a child is able to detect when aided by his/her hearing aid. From this information, an audiologist is able to determine whether a young child is adequately fit with an appropriate hearing device for the purposes of hearing the softest elements of speech that will permit the child of developing an acceptable level of language. While it is possible to obtain the aided thresholds from older children using traditional behavioral methods, it is impossible to obtain this information from very young children. While it would be possible to measure aided threshold using an electrophysiological technique such as ABR and ASSR, these would entail delivering acoustic stimuli in the free field. Free field measurements possess inherent limitations when testing infants and toddlers, such as ensuring a fixed positioning of the head and torso during the entire duration of the measurement; a task quite difficult to achieve in the pediatric population. It would be very useful for an objective, electrophysiological methodology to assist with the estimation of aided thresholds in this capacity, without the limitations imposed by free-field measurements.
When applying an electrophysiological method for measuring the hearing threshold of a baby or a young child, it should preferably be asleep; otherwise signals may arise due to neuron-muscular activity, and interfere with the auditory evoked potentials sought. As such, the measurements may not reflect the supposed evoked auditory responses. Furthermore, when an electrophysiological threshold measurement is carried out in a free field, there is an uncontrolled variation of about 10 dB to 20 dB.